The present invention relates to a method of manufacturing a printed circuit board having inner via-holes.
In recent years, due to the miniaturization and multiple functions of electronic equipment, printed circuit boards have been increasingly becoming multi-layered and higher in component density.
Generally, a method of manufacturing a printed circuit board comprises a step of alternately laminating a plurality of boards with conductor circuits and adhesive sheets (usually called xe2x80x9cprepregxe2x80x9d), a step of press-fitting under heat, followed by a step of making a through-hole therein, and a step of making electrical connections between surface and inner layers by means of copper plating or the like in the through-hole.
However, with increase in demand for video movie cameras and mobile communication equipment or the like, printed circuit boards used for these cameras and equipment have been required to be smaller in size and higher in density. To meet such requirement, there is a proposal of a method for manufacturing a printed circuit board disclosed in Japanese Laid-open Patent No. 6-268345. The method of manufacturing a printed circuit board in Japanese Laid-open Patent No.6-268345 comprises a step of making a through-hole in a non-compressible porous substrate having release film on either side thereof, a step of filling conductive paste into the through-hole, a step of electrically connecting both sides of the board to each other by sticking metallic foil on either side of the porous substrate and pressurizing same under heat after peeling the release film, and a step of patterning by itching the metallic foil to form a circuit.
A conventional method of manufacturing a printed circuit board will be described in the following with reference to the drawings. FIG. 3 to FIG. 5 show cross-sectional views of manufacturing steps for a conventional printed circuit board. First, as shown in FIG. 3(a), release film 12 is disposed on either side of porous substrate (hereinafter referred to as xe2x80x9cprepreg sheetxe2x80x9d) 11. The porous substrate 11 has a square plane, which is 500 mm in side length and t1 (for example, about 150 xcexcm) in thickness. The release film 12 includes polymer film such as PET (polyethylene terephthalate) of 19 xcexcm thick, having a silicone-based release layer disposed on one side thereof. Prepreg sheet 11 used is a composite material consisting of non-woven cloth of aromatic polyamide fiber and thermosetting epoxy resin with which the non-woven cloth is impregnated.
Next, as shown in FIG. 3(b), through-hole 13 is formed in a predetermined portion of the prepreg sheet 11 by laser operation.
Subsequently, the prepreg sheet 11 is placed on a table of a printing machine (not shown), and conductive paste 14 is printed on the release film 12. Then, as shown in FIG.(c), the conductive paste 14 is filled into the through-hole 13. In this case, the release film 12 serves to prevent staining of a printing mask and prepreg sheet 11.
Next, as shown in FIG. 3(d), the release film 12 on either side of the prepreg sheet 11 is removed at the room temperature. And, as shown in FIG. 3(e), metallic foil 15 such as copper foil is stuck on either side of the prepreg sheet 11 and is pressurized under heat. In this way, as shown in FIG. 3(f), the prepreg sheet 11 and the metallic foil 15 are adhered to each other, and the prepreg sheet 11 is compressed to t2 (for example, about 100 xcexcm) in thickness (t2 greater than t2), then the metallic foils 15 on either side of same are electrically connected to each other by conductive paste 14. At that time, epoxy resin, that is one of the components of the prepreg sheet 11, and the conductive paste 14 become hardened.
And, as shown in FIG. 3(g), the metallic foil 5 is subjected to patterning by photolithography, followed by etching, and thereby, circuit pattern 16 is formed on either side of the prepreg sheet 11.
However, in the conventional configuration described above, through-holes in a prepreg sheet are reduced in diameter as a result of increasing in density of the circuit pattern, and after filling conductive paste, when the release film is removed from the prepreg sheet 11, the conductive paste sticking to the release film is removed from the prepreg sheet along with the release film. A possible cause of this problem is that reducing only the through-hole diameter by using conventional release film mainly to meet the requirement for reducing the through-hole diameter causes the through-hole diameter to be excessively reduced as compared with the thickness of the release film, that is, the aspect ratio (the ratio of the diameter of the hole formed in the release film to the thickness of the release film) becomes greater. Accordingly, it is not possible to make the conductive paste projected in convexity from the surface of the prepreg sheet after removal of the release film. If the worse comes to the worst, as shown in FIG. 4(d), the conductive paste will be concavely curved from the uppermost surface of the prepreg sheet, thereby forming a depletion layer 18. Consequently, it becomes difficult to make electrical connections between the conductive paste and the metallic foil in the following steps.
Also, as shown in FIG. 5, for preventing removal of the conductive paste during release film peeling operation, it is possible to lower the aspect ratio by using thinner release film. However, when thinner release film 22 is used, the amount of conductive paste projected from the prepreg sheet will become less. As a result, the absolute amount of conductive paste that should exist in the through-hole after compression by pressure application becomes insufficient. Accordingly, the connection resistance between the metallic foil and the conductive paste will increase.
A method of manufacturing a printed circuit board of the present invention comprises:
(a) a step of disposing a first release film on a surface of a substrate, and disposing a second release film on a back surface of the substrate;
(b) a step of forming a through-hole through the first release film, the second release film and the substrate;
(c) a step of filling conductive paste into the through-hole;
(d) a step of peeling the first release film and the second release film from the substrate with the through-hole filled with the conductive paste;
(e) a step of placing a first metallic member on the surface of the substrate with the release films removed, and placing a second metallic member on the back surface of the substrate;
(f) a step of compressing under heat the substrate with the first metallic member and the second metallic member disposed on either side thereof; and
(g) a step of forming a desired circuit pattern on the first metallic member and the second metallic member,
wherein the thickness of the first release film is different from the thickness of the second release film;
when the first release film is peeled, a first projected paste portion is formed, which is projected from the surface of the substrate having the through-hole, and a second projected paste portion is formed, which is projected from the back surface of the substrate having the through-hole; and
the first metallic member is electrically connected to the first projected paste portion, and the second metallic member is electrically connected to the second projected paste portion.
Preferably, the thickness of the first release film is greater than the thickness of the second release film, the thickness of the first projected paste portion depends upon the thickness of the first release film, the thickness of the second projected paste portion depends upon the thickness of the second release film, and the thickness of the first projected paste portion is greater than the thickness of the second projected paste portion.
Preferably, a step of forming the through-hole is performed by application of a laser beam, and the laser beam is applied from the first release film side being greater in thickness toward the first release film, the substrate, and the second release film. In this way, a first through-hole in the first release film is formed larger in diameter than a second through-hole in the second release film.
With the above configuration, the conductive paste is disposed on the substrate in a manner such that the first projected paste portion and the second projected paste portion being desirable in thickness are projected from the substrate. And, each of the metallic members is precisely electrically connected to the conductive paste filled in the through-hole. As a result, the circuit pattern disposed on the surface of the substrate is precisely electrically connected to the circuit pattern disposed on the back of the substrate, thereby preventing increase in electrical resistance between the circuits. Further, even in case the printed circuit board has a small-diameter through-hole corresponding to a high-density circuit pattern, the conductive paste may be reliably filled into the through-hole. As a result, it is possible to make reliable electrical connections between the metallic member on either side of the substrate and the conductive paste.